Anthrax spores can germinate, grow and reproduce in soil

SEATTLE - For Bacillus anthracis, the bacterium that causes anthrax, nothing beats the inside of a warm human or animal host for triggering an intense spurt of rapid growth and reproductive activity. But when a warm-blooded animal isn't available, new research by scientists in the University of Michigan Medical School shows that ordinary dirt can do the job, as well.

U-M microbiologist Philip C. Hanna, Ph.D., announced his research findings in a Feb. 14 presentation at the American Association for the Advancement of Science meeting. His results are surprising, because most scientists believe B. anthracis spores will not germinate and grow unless they are inside a living host.

During the AAAS symposium on how environmental organisms cause disease, Hanna described what happened when he seeded soil samples with anthrax spores, added water and let the mixture incubate in the laboratory.

"All stages of the anthrax life cycle were found to occur in soil, including germination of spores, bacterial reproduction and formation of new spores," said Hanna, an assistant professor of microbiology and immunology in the U-M Medical School. "Our research demonstrates that anthrax can complete its full life cycle without a mammalian host."

Researchers in Hanna's laboratory collected ordinary soil from the bank of Miller's Creek - part of the Huron River watershed - in Ann Arbor, Mich. The soil was taken to the laboratory where U-M researchers added water and dormant spores of an attenuated, or non-infectious, strain of B. anthracis, which was modified to make it safe to handle in university laboratory facilities. Then, U-M scientists cultured the soil samples to see whether the spores would germinate and grow.

"The spores germinated and continued to replicate until they ran out of nutrients in the soil," Hanna reported. "At that point, the bacteria formed new spores and became dormant. In every case, we ended up with more spores than we added to the original soil samples."

Hanna emphasized that the Ann Arbor soil samples did not contain naturally occurring anthrax spores, and that the attenuated strain of anthrax used in the study is unable to infect or produce disease in people.

Scientists have detected dormant anthrax spores in soil before, but vegetative or "growing" anthrax has never been found in nature outside a mammalian host. According to Hanna, this led most scientists to assume that B. anthracis had to get inside a person or an animal in order to reproduce.

"We questioned that assumption and decided to test it," Hanna said. "There have been several cases of disease outbreaks in livestock when a longer-than-normal rainy season was followed by a hot, dry period. While there are no scientific studies to confirm whether this curious finding occurs naturally in the environment, our research suggests that the potential exists."

Even though the B. anthracis life cycle inside the host has received increased scientific attention recently, Hanna says researchers still have a lot to learn about how it germinates and reproduces outside the host.

In his AAAS presentation, Hanna compared what happens during the stages of anthrax infection in mammalian cells to what happens when the bacterium grows in soil.

"B. anthracis has an amazing ability to detect a wide range of molecular signals from its environment, and then turn on the right subset of genes to maximize its energy," Hanna said. "Because there is no host immune system to fight, anthrax in soil doesn't need to activate its full complement of genes - many of which are used to evade the immune response in a living host," Hanna said. "Also, since there are fewer nutrients in soil than in the body, the increase in bacterial numbers is slower and not as dramatic."

Hanna's research suggests that this deadly pathogen may be even more versatile and resilient than scientists originally believed. "If our new findings accurately reflect the ecology of anthrax, it bodes very poorly on any potential ambitions we might have to eradicate its spores from the environment," he said.

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Collaborators on the soil study included Nicholas H. Bergman, Ph.D., a U-M research investigator, and Brendan Thomason, a U-M graduate student.

Hanna is part of a collaboration of scientists from the University of Michigan, The Institute for Genomic Research (TIGR) and The Scripps Research Institute who are working together to identify all the genes and proteins involved in the deadly transformation of B. anthracis from a dormant spore to an active, growing organism. The collaboration focuses on the growth and development of anthrax inside a living host.

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